Broadband communications access, on which our society and economy is growing increasingly dependent, is now becoming available to users on board mobile platforms such as aircraft, ships, automobiles and trains. Broadband communication services for passengers of the mobile platform include Internet access, movies on demand, rebroadcast live television, virtual private network access and other interactive and real time services. While the technology exists to deliver the broadband communication services to mobile platforms, conventional communications systems have been commercially unacceptable for various reasons including the relatively high cost of service and/or low data rates.
Communication systems for mobile platforms usually include satellites having a footprint that covers a specific region of the earth that requires service. For communications systems with geosynchronous satellites, the footprint covers a relatively fixed region of the earth. For satellites with low and medium earth orbits, the footprints cover a moving region, in other words the footprints sweep across the earth. Ground stations send data to the mobile platforms through a bent pipe via one or more satellites. The mobile platforms, in turn, send data to the ground stations via the satellite. The ground stations provide the mobile platform with links to the Internet, private networks, and/or other servers and services.
Referring now to FIG. 1, a satellite 10 covers a region of the earth that includes multiple mobile platforms 12. The mobile platforms 12 can be airplanes that are depicted in FIG. 1, other air-based mobile platforms or ground-based mobile platforms such as trains, ships and automobiles. The satellite 10 relays a forward link 20 from the ground station 16 to the mobile platforms 12. The satellite 10 relays a return link 22 from the mobile platforms 12 to the ground station. The forward link 20 received by the mobile platform 12-1 may have different radio frequency (RF) power losses than the forward link 20 to the mobile platform 12-2.
The different RF power losses of the mobile platforms are due to space loss and/or other losses. Space losses occur at the periphery of the satellite beam as the distance between the satellite 10 and the mobile platform 12 increases. The type of antenna, the pointing accuracy of the antenna and the mechanical orientation of the antenna with respect to its radiation pattern may also impact RF loss of the mobile platform 12. Some link losses depend on the variations that occur during manufacturing of the satellite communications hardware. Other link losses depend on environmental conditions and on region-specific interference such as weather, electromagnetic interference, and/or thermal noise.
Both the cost of service and the speed of conventional communication systems are adversely impacted by the requirement that the data rate of the satellite 10 is limited to the data rate supported by the weakest communication link. For example, the mobile platform 12-2 is capable of receiving data at 1 Mbps with a small link margin due to a storm between mobile platform 12-2 and the satellite 10. The mobile platform 12-1, however, that is not hindered by the storm must also receive data at 1 Mbps. The mobile platform 12-1 has greater than 6 dB of link margin. Therefore, the mobile platform 12-1 is capable of receiving data at a much higher data rate. In other words, the mobile platform 12-1 has a significant amount of unusable link margin.